This disclosure relates generally to a sensor for measuring a respiration.
During apnea there is no movement of the muscles of respiration and the volume of the lungs initially remains unchanged. Depending on the openness of the airways there may not be a flow of breathing gas between the lungs and the ambient during apnea. Apnea can be drug-induced (e.g., opiate toxicity), mechanically induced (e.g., strangulation or choking), or it can occur as a consequence of neurological disease or trauma as was explained above.
Many people in hospital wards suffer from apnea, caused by opiates or other medicine. Patients may also have obstructive apnea, which is caused by a blockage in the airways, such as thong or similar physical obstruction. Patients usually don't have any monitoring connected on as it is rather expensive and there are usually many patients in the ward. Furthermore, there is also shortage of nursing personnel keeping an eye on patients continuously.
Many healthy people suffer from sleep apnea that can be divided into three distinct forms: central, obstructive, and a complex sleep apnea, which is a combination of central and obstructive apnea. In central sleep apnea, breathing is interrupted by the lack of respiratory effort. In obstructive sleep apnea, breathing is interrupted by a physical block to airflow despite respiratory effort. In complex sleep apnea, there is a transition from central to obstructive features during the events themselves. Central apnea is commonly caused by neurological etc. characteristics, whereas obstructive apnea is caused by for example over weight.
Many elderly people at home have to take lot of different medical pills or other type of medicine, which may cause apnea. Also many infants, small babies and even small children up to three years age suffer from sudden infant death syndrome (SIDS). It is also very stress full for many parents, as they can not sleep well during nights, since they compulsively wake up to see how their small babies and children are sleeping.
Currently there does not exist a device that can reliably measure the actual breathing gas flow through the nose or the mouth and can reliably detect apnea. The standard definition of apnea is cessation of inspiratory gas flow for 20 seconds, or for a shorter period of time if accompanied by bradycardia (heart rate less than 100 beats per minute), cyanosis, or pallor. Most devices, such as impedance measurement, resistive belts, or piezo-resistive belts etc., does not measure the actual flow of breathing gas through the nose or the mouth, but they measure for example the respiratory muscle movement or chest movement. These indirect measurements are unreliable since the respiratory muscle or chest movement may occur even when the patient is suffering from apnea. Devices are also sensitive to motion artefacts and other disturbances. Some devices try to measure the actual breathing gas flow through the nose or the mouth. Devices, based on sensing the breathing gas flow or the pressure, are very sensitive to mechanical motion and vibrations as well as surrounding air flowing by, causing disturbances and artefacts. Devices based on measuring the thermal flow of breathing gas are less sensitive to motion, but they are very sensitive to surrounding air flowing by the device that causes signal disturbances and artifacts. Some of them are also very sensitive to false skin contact that decreases the sensitivity or even destroy the measurement.
Disposability is one of the clinical requirements for the device that is in close contact to patient's airways, since existing cleaning practices are not reliable enough to ensure high enough level of purity for reusable devices. Contaminated reusable devices easily cause a risk of cross contamination between different patients, who already have a lowered level of immunity against bacteria and viruses. Existing devices are rather expensive and thus they are usually reusable.
Existing apnea sensors based on measuring the thermal flow of the breathing gas usually contain two or three thermistors. It is common to place two thermistors into both hawse pipes and one in to the front of the mouth. Some devices have one thermistor placed under the nose and one in front of the mouth. In most of the devices thermistors are suspended in to open air and they are in straight connection with interfering air flowing by the thermistors or thermistors may even be in straight contact with patient's skin. Such devices suffer from signal disturbances and lowered or no signal sensitivity as the thermal mass is increased by the patient's body. Some devices may have housing covering the thermistors to minimize signal disturbances caused by the flowing surrounding air or skin contact. However, the housing forms a continuous cavity between the mouth and the nose also connecting the thermistors into one common gas flow path. When the patient breathes through the nose and the mouth at the same time, there is no gas flow through the cavity and no gas flow by the thermistors. Obviously, the measurement signal would be zero or something that is not proportional to respiration flow and respiration rate. When the patient breaths through the nose or the mouth the same air flows back and forth in the channel causing flow disturbances and error to the measurement and in the worst case it decreases the patient safety as the patient re-breaths gases, which causes decreased gas exchange in the lungs.